Ion implantation systems are used to dope semiconductors with impurities in integrated circuit manufacturing. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam of desired energy. The ion beam is then directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the wafer by airborne particles. A typical ion implanter includes an ion source for generating the ion beam, a beamline system including mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer to be implanted by the ion beam. For high energy implantation systems, an acceleration apparatus is provided between the mass analysis magnet and the target chamber for accelerating the ions to high energies.
In order to achieve a desired implantation for a given application, the dosage and energy of the implanted ions may be varied. The ion dosage controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dosage applications. The ion energy is used to control junction depth in semiconductor devices, where the energy levels of the beam ions determine the degree of depth of the implanted ions. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. In addition, the continuing trend toward higher device densities on a semiconductor wafer requires careful control over the uniformity of implantation beams being scanned across the workpiece.
Another continuing trend is toward larger and larger semiconductor wafer sizes, such as 300 mm diameter wafers. Coupled with higher device densities, the larger wafer size increases the cost of individual wafers. As a result, control over implantation uniformity and other parameters is more critical than ever in avoiding or mitigating the cost of scrapping wafers. In many ion implantation systems, a small ion beam (e.g., a pencil beam) is imparted onto a wafer target through mechanical and/or magnetic scanning, in order to provide the desired implantation. The ion beam is shaped according to the ion source extraction opening and subsequent shaping apparatus, such as the mass analyzer apparatus, resolving apertures, quadrupole magnets, and ion accelerators, by which a small ion beam is provided to the target wafer or wafers. The beam and/or the target are translated with respect to one another to effect a scanning of the workpiece. Batch implanters provide for simultaneous implantation of several wafers, which are rotated through an ion beam path in a controlled fashion. Serial implanters, on the other hand, provide implantation of a single wafer at a time.
Where a small ion beam is used, the serial implanters provide a relatively complex target scanning system to impart the beam across the wafer in a uniform manner. For example, mechanical translators are provided to translate the wafer in a single axis, while magnetic apparatus are provided to scan the beam in a perpendicular axis to achieve a raster type scanning of the wafer surface. However, in order to reduce the complexity of such implantation systems, it is desirable to reduce the cost and complexity of target scanning systems, and to provide for elongated ribbon-shaped ion beams. For a ribbon beam of sufficient longitudinal length, a single mechanical scan may be employed to implant an entire wafer, without requiring additional mechanical or magnetic raster-type scanning devices. Such a beam may be employed with serial as well as batch type target scanning systems. However, where a ribbon beam is used in such a single-scan system, it is necessary to ensure that the beam is uniform across the width, in order to provide for uniform implantation of the wafer or wafers. In some prior systems, a small ion beam is mass analyzed, and then collimated to provide a mass analyzed ribbon beam for implanting wafers. Such systems, however, suffer from difficulties in providing a high current beam at low energies due to the high beam density associated therewith, wherein the high beam density tends to result in beam blow-up due to space charge. Accordingly, it is desirable to provide improved ion implantation apparatus and methodologies by which uniform ribbon beams may be provided for implanting semiconductor wafers.